Gerard Leyson

Quantitative prediction of solute strengthening in aluminium alloys

Despite significant advances in computational materials science, a quantitative, parameter-free prediction of the mechanical properties of alloys has been difficult to achieve from first principles. Here, we present a new analytic theory that, with input from first-principles calculations, is able to predict the strengthening of aluminium by substitutional solute atoms. Solute-dislocation interaction energies in and around the dislocation core are first calculated using density functional theory and a flexible-boundary-condition method. An analytic model for the strength, or stress to move a dislocation, owing to the random field of solutes, is then presented. The theory, which has no adjustable parameters and is extendable to other metallic alloys, predicts both the energy barriers to dislocation motion and the zero-temperature flow stress, allowing for predictions of finite-temperature flow stresses. Quantitative comparisons with experimental flow stresses at temperature T=78 K are made for Al-X alloys (X=Mg, Si, Cu, Cr) and good agreement is obtained.

Lattice Distortions in the FeCoNiCrMn High Entropy Alloy Studied by Theory and Experiment

Lattice distortions constitute one of the main features characterizing high entropy alloys. Local lattice distortions have, however, only rarely been investigated in these multi-component alloys. We, therefore, employ a combined theoretical electronic structure and experimental approach to study the atomistic distortions in the FeCoNiCrMn high entropy (Cantor) alloy by means of density-functional theory and extended X-ray absorption fine structure spectroscopy. Particular attention is paid to element-resolved distortions for each constituent. The individual mean distortions are small on average, <1%, but their fluctuations (i.e., standard deviations) are an order of magnitude larger, in particular for Cr and Mn. Good agreement between theory and experiment is found.

Friedel vs. Labusch: the strong/weak pinning transition in solute strengthened metals

Abstract Solute strengthening in substitutional alloys can be classified into two categories, strong-pinning (Friedel) and weak-pinning (Labusch), each with its own characteristic scaling with concentration and temperature. The transition between the two strengthening mechanisms as a function of solute concentration has previously been estimated at zero temperature. Here, the transition is investigated more completely, using a new model for the Labusch-type weak pinning model and considering finite temperature and strain rate. A parametric study of the transition as a function of solute concentration and dislocation core structure shows that the temperature dependence of the transition concentration greatly depends on the dislocation core structure and, in general, can differ significantly from the zero-temperature value. The zero-temperature transition concentration itself also differs from the standard estimate. Except for the most highly localized cores, the Labusch model controls the strengthening for concentrations greater than and temperatures greater than K. Using dislocation core structures and solute/dislocation interaction energies derived from first-principles, the transition concentration at finite temperature for Al–X (X = Mg, Si, Cu, Cr, Mn and Fe) and Mg–Al (basal) alloys is also predicted to be in the range of . Overall, at temperatures and concentrations relevant to engineering applications, the Labusch model is therefore expected to control the strengthening mechanisms for these alloys and for most dislocation core structures in metals.

Solute strengthening at high temperatures

The high temperature behavior of solute strengthening has previously been treated approximately using various scaling arguments, resulting in logarithmic and power-law scalings for the stress-dependent energy barrier Delta E(tau) versus stress tau. Here, a parameter-free solute strengthening model is extended to high temperatures/low stresses without any a priori assumptions on the functional form of Delta E(tau). The new model predicts that the well-established low-temperature, with energy barrier Delta E-b and zero temperature flow stress tau(y0), transitions to a near-logarithmic form for stresses in the regime 0.2 < tau/tau(y0) <= 0.5 and then transitions to a power-law form at even lower stresses tau/tau(y0) < 0.03. Delta E-b and tau(y0) remains as the reference energy and stress scales over the entire range of stresses. The model is applied to literature data on solution strengthening in Cu alloys and captures the experimental results quantitatively and qualitatively. Most importantly, the model accurately captures the transition in strength from the low-temperature to intermediate-temperature and the associated transition for the activation volume. Overall, the present analysis unifies the different qualitative models in the literature and, when coupled with the previous parameter-free solute strengthening model, provides a single predictive model for solute strengthening as a function of composition, temperature, and strain rate over the full range of practical utility.